The present invention relates to reduction gears, and relates more particularly to such a gear structure for a reduction gear which comprises an internal gear and an external gear meshed together and moved relative to each other through a rotary motion, wherein the radius of the root arcs of the teeth of the external gear is M, which is the ratio between the diameter of the crest and the number of teeth, and is the line of arc in tangent with the crest arcs of each two adjacent teeth; the tooth form curve of the external gear is obtained by connecting all lines of arc; the tooth form curve of the internal gear is obtained by connecting all engaging points at the crest arcs and root arcs of the external gear.
Reduction gears are intensively used in machinery for power output transmission. Conventional reduction gears commonly use planet gears to reduce the speed of output power in the same direction. These reduction gears commonly comprised of an internal gear and an external gear of different numbers of teeth meshed together and moved relative to each other through an eccentric rotary motion. However, using planet gears to reduce the speed of output power in the same direction has drawbacks. Because a planet gear achieves rotary motion by means of the engagement of gear teeth, friction loss between contact surfaces affects the working efficiency of the mechanism. If the number of teeth of the internal gear is close to that of the external gear, an interference tends to occur, therefore the application is limited. In order not to occupy much installation space, the thickness of the root of each tooth must be limited to a certain value, which results in weak structural strength of the teeth, and therefore the teeth of the gears tend to be broken. Furthermore, because the precision of the specifications of the teeth is critical, the processing process of the gears is complicated.